Electrical capacitors are used for storing energy in a variety of applications. Operating voltages for such capacitors range from a few volts (e.g. miniature or micro electronic circuitry) to thousands of volts (e.g. power utility applications). A capacitor comprises a pair of conductive plates or electrodes separated by a dielectric material. The electrodes are typically composed of copper, silver, aluminum foils or vacuum deposited zinc or aluminum. Capacitors utilize a variety of dielectric materials ranging from ceramics, metal oxides, plastic sheets or films to paper.
Higher voltage capacitors are generally constructed of multiple sheets of a dielectric material such as polypropylene or polyester film in between sheets of foil such as aluminum foil. These materials are typically wound into a roll and vacuum impregnated in roll form or collapsed into rectangular elements and then vacuum impregnated. Capacitors intended to operate at voltages greater than 600 V are normally completely impregnated with a low viscosity dielectric-liquid with good gas absorbing properties.
It is important for the operation of high voltage capacitors that all void spaces be filled. Otherwise there will be corona breakdown of voids, which will lead to low breakdown strength and failure. Thus, it is known to impregnate such capacitors with a liquid impregnant such as mineral oil, castor oil, polybutylene, dioctyl phthalate and other liquid impregnants. It is also known to impregnate such capacitors with epoxy and urethane solid dielectric materials. The impregnants generally fill the void spaces in the capacitor to increase capacitance, reduce corona discharges and aid in the transfer of heat from the capacitor to the outside environment.
On the one hand, it is desirable to use an impregnant that is not liquid so that there will be no leaking of fluid, with its possible negative environmental impact, in the event of a capacitor failure or in case the capacitor case is not well sealed. However, voids can be created in a solid impregnant when it cures. Moreover, voids can develop more readily with a solid impregnant during thermal cycling of a capacitor in service, where temperatures can range between -55.degree. C. and 85.degree. C. In particular, low temperatures can cause solid capacitor materials to contract, which can open up voids in the capacitor. If these voids are adjacent to a foil edge where there is a high electric stress, there can be a gas void breakdown at high voltages that would cause the capacitor to fail at unusually low breakdown voltages. Thus, it is not common to use solid or gel materials as impregnating materials for high voltage capacitors.
Rather, despite environmental concerns, high voltage capacitors are commonly impregnated with a low viscosity liquid with good gas absorption properties. The liquid is better able to expand and contract along with the dissimilar capacitor materials during thermal cycling to avoid formation of voids within the capacitor. Yet, it would be desirable to have a gel impregnant that is flexible enough and adherent enough to the capacitor materials to prevent voids from occurring. It would also be desirable to have a dielectric gel impregnant with good insulating properties, high dielectric constant and good compatability with capacitor materials.
The normal impregnation process begins by placement of capacitors to be impregnated in a chamber that is then evacuated. The vacuum chamber is flood filled with the impregnant so that the capacitors inside the chamber are covered. Thereafter, the chamber is exposed to atmospheric pressure, which aids in forcing the impregnant into any voids in the capacitor. While this method is satisfactory for low viscosity liquid impregnants, it does not always work well with higher viscosity materials or gels. Accordingly, an improved, more reliable process for impregnation would be desirable.